LONDON, England (CNN) -- An industrial vacuum cleaner left switched on inside the historic Cutty Sark ship was the most likely cause of the fire last year that gutted the 19th-century tea clipper, police said Tuesday.
The blaze gutted the historic vessel, causing millions of dollars' worth of damage.
The vacuum cleaner was being used to remove waste from the vessel as it underwent an extensive three-year $50-million restoration project. Police said their investigation showed the vacuum had inadvertently been left running for 48 hours before the fire broke out.
In their report, issued Tuesday, police said the fire started accidentally and there was no evidence the fire was caused by arson.
The fire began before dawn on Monday, May 21, 2007 and took more than 40 firefighters several hours to extinguish. It reduced the Cutty Sark, a popular London attraction, to a skeleton.
The clipper -- a very fast multi-masted sailing ship used for transporting high-value goods such as tea and wool -- was once regarded as the pinnacle of merchant sail vessel development.
Investigators on Tuesday detailed a series of fire safety problems at the Cutty Sark site that they said increased the possibility of fire. Staff failed to follow a fire safety plan that called for regular fire alarm checks and a daily inspection with a fire marshal, police said.
"We believe that lack of this fire check on Friday, 18th of May, is a significant factor in the fire," said Dave Garwood, the lead investigator in the case for London's Metropolitan Police. "It was a missed opportunity to prevent the fire, in our view."
The fire safety plan stipulated that two guards were to be on duty at the ship overnight, with hourly patrols of the interior until their shift ended at 7 a.m., police said. But the guards on duty that night, Garwood said, weren't doing their jobs.
"We discovered that they had falsified their occurrence book by prewriting entries up unto and including 7 a.m. on the 21st of May, 2007, with the entry booked, 'Off duty, all is in order,'" Garwood said. "The page bearing this entry was then torn from the book once the fire had clearly started and a clumsy attempt was made to hide it in the wastebin.
"It appears that one of the guards had intended to leave at 5 a.m.," Garwood said. "It is not unfair to conclude that the fire could have been detected earlier had they patrolled correctly."
Physical evidence and surveillance camera footage show the fire started at the rear of the ship's lower deck, police said, where the vacuum was among several items of electrical equipment being used during restoration.
Fire investigator Nick Carey of the London Fire Brigade said witnesses reported previous problems with the vacuum. It had overheated in the past, the witnesses said, and had already been sent to the supplier once for repair.
A week before the fire, an employee discovered the vacuum had been left running overnight, Carey said. Witnesses said other equipment had also been left plugged in in the past.
Officials with the Cutty Sark Trust, which is in charge of the restoration, did not respond to details in the report but thanked the police and fire services for conducting the investigation.
"It could have been so much worse," said Richard Doughty, chief executive of the trust. "We came very close to losing the ship altogether."
He said only a small percentage of the ship's original fabric was lost. Restoration work will continue, he said, even though the fire delayed completion by about two years to the summer of 2010.
"The overall cost, because of the extensive cleanup, redesign, and the need for replacement materials, has increased substantially," Doughty said. "The fire added approximately £10 million ($18.1 million) to the project costs, but we now need in excess of £3 million ($5.4 million) to complete the final presentation of the ship."
The Cutty Sark had been on display since the 1950s. More than 15 million people had visited the ship in its dry dock at Greenwich, a World Heritage Site on the southern bank of the River Thames.
The vessel rose to prominence after leaving London on its first voyage February 16, 1870, and sailing around Cape Hope to Shanghai. Capable of reaching speeds of up to 17.5 knots and covering more than 360 miles a day, it outstripped most other vessels.
In 1885, the 85-meter (280 feet) ship achieved a record-breaking wind-powered voyage from Australia to England, completing the distance in 72 days. But its glory days were short-lived as steam ships quickly came to replace sail.
Doughty has called the Cutty Sark "the epitome of speed under sail."